Ammonia Production Via Nitrogen-rich Microalgae Pyrolysis/Gasification Process With Iron-based Perovskite Catalyst

Ammonia（NH₃）,one of the most significant chemical feedstocks,is endowed with another role as a potential energy carrier.The fossil fuel-based Haber-Bosch ammonia synthesis industry is an energy-intensive process.It is thus highly desirable to explore innovative ammonia production methods driven by renewable energy sources.Microalgae,the carbon-neutral and renewable resource with a high content of nitrogen,might be a feasible source of renewable nitrogen and hydrogen.During thermo-chemical conversion processes,nitrogen in microalgae can be converted into a large quantity of NH₃ at ambient pressure.Ammonia production from microalgae pyrolysis/gasification,offers a mild and sustainable route to produce green ammonia.In order to improve NH₃ production,it is crucial to adjust the transformation and migration of N-containing compounds into NH₃.In this work,microalgae was chosen as the feedstock,the mechanism of nitrogen transformation and NH₃formation,as well as the directional control and catalytic mechanism on NH₃ production were carried out.The main results were shown as follows.The sufficiency of H radical is an essential factor determining NH₃ yield during thermochemical process.Mechanisms of nitrogen transformation and NH₃ formation during microalgae pyrolysis under H₂-rich atmosphere were investigated in depth with exploration of N-containing products under variant temperature.Results indicated that nitrogen in microalgae was primarily in the form of protein-N（～89.2%）with some inorganic-N.At400～600°C,inorganic-N cracked once pyrolysis began.Subsequently,protein-N cracked and generated pyridinic-N and pyrrolic-N in biochar via cyclization or dimerization reactions,and part of the resulting pyridinic-N further converted to quaternary-N via ring condensation reactions.Meanwhile,amide-N,nitrile-N and N-heterocyclics（pyridines,pyrroles,and indoles）were formed in tar-N.NH₃ was mainly produced from the decomposition of inorganic-N,the cracking of labile protein-N,and the deamination of amide-N at low temperatures（400～600°C）.H₂ was beneficial for the thermal cracking of stable proteins and the deamination of amide-N,thereby facilitating the formation of NH₃.The increments of NH₃ at high temperatures（700～800℃）were linked with the secondary reactions of tar-N and char-N.Specifically,contributions on NH₃ yield were attributed to the hydrogenation of N-heterocyclics and the deamination reactions of amide-N.H₂ could provide abundant H radicals,which were absorbed on the N-sites on char surface,promoting the ring-opening of N-heterocyclic species to generate more NH₃.Based the mechanism of nitrogen transformation and NH₃ formation obtained,LaFe O₃perovskite and H₂ atmosphere were combined to enhance NH₃ production during microalgae pyrolysis.The distribution and conversion of N-containing functionalities in biochar,bio-oil and gas products were evaluated.Synergistic effects between H₂ and LaFe O₃ on nitrogen conversion during microalgae catalytic pyrolysis were revealed.Results indicated that the synergistic effects between H₂ and LaFe O₃ significantly increased gas-N yield,while decreased char-N and tar-N yield.H₂ and LaFe O₃not only favored the conversion of protein-N to pyridinic-N,pyrrolic-N,and quaternary-N in char,but also accelerated the deamination of amides,pyrroles,and pyridines,thus facilitating the formation of NH₃.Pyrolysis temperature played a considerable role in distribution and conversion of N-species.Increasing temperature could promote the fuel-nitrogen transfer into gas phase.With the temperature increasing from 400°C to 800°C,NH₃ yield increased from 27.32 wt.%to 47.40 wt.%.Ex-situ catalytic pyrolysis could avoid the limit of heat and mass transfer,thus facilitating the cracking of macromolecules of N-containing intermediates into more light nitrogenous gases.Compared to in-situ,ex-situ catalytic pyrolysis had a more significant effect on improving NH₃ yield.Unlike pyrolysis,steam is generally beneficial as the gasifying agent for driving N migrating from the solid and liquid phase to gas phase during steam gasification.Three amino acids,glutamic acid（Glu）,glycine（Gly）,and phenylalanine（Phe）,with distinct structures were screened as N-containing model compounds to assess the effects of steam on nitrogen chemistry during gasification process in depth.Results indicated that chemical structure had a significant impact on thermal decomposition properties and N-conversion selectivity of amino acids.Gly possessed the highest NH₃-N yield,because the simple amino structure easily detached as NH₃.Phe,as the representative of aromatic amino acids,showed a higher HCN yield.NH₃/HCN ratio of model amino acids during gasification process were confirmed to be with a sequence of Gly>Glu>Phe.The introduction of steam could provide abundant H radicals.On the one hand,steam and H radicals could strengthen the reforming of tar-N and the gasification of char-N,thus facilitating the formation of N-containing gases.On the other hand,steam and H radicals played a crucial role in secondary gas-phase reactions,and drived the conversion of HCN into NH₃,which would promote the selectivity of N-conversion into NH₃.The NH₃ production behavior during pyrolysis,catalytic pyrolysis,steam gasification,and steam catalytic gasification were experimentally investigated.The effect of steam content,gasification temperature and cycle number on nitrogen distribution and NH₃ production were demonstrated.Furthermore,the synergy of steam and LaFe O₃ on nitrogen conversion was proposed.Results indicated synergistic effects of steam and LaFe O₃ were able to promote the decomposition of proteins,the hydrogenation of char-N,and the deamination of amides,resulting in a remarkable release of NH₃.More importantly,steam and LaFe O₃ could significantly promote the conversion of HCN into NH₃,which would enhance the selectivity of NH₃.With the increasing steam content from 0.3 m L/min to 0.7 m L/min,proper steam content provided considerable amounts of H radicals for enhanced NH₃ production,whereas excessive steam was likely to facilitate the NH₃ conversion to H₂.Gasification temperature exerted positive effects on the NH₃ yield at 700～800°C,but further increase in temperature caused a reduction in NH₃ production,as a result of its decomposition into N₂.The highest conversion of nitrogen into NH₃（55.98 wt.%）was obtained at 800°C with a steam feeding rate of 0.7 m L/min and the catalyst-to-feedstock ratio of 1.La_1-xCa_xFe O₃ perovskites（x=0,0.1,0.2 and 0.3）are proposed as potential catalysts to promote NH₃ selectivity.The catalytic performance of La_1-xCa_xFe O₃ perovskites in terms of activity and stability was investigated for ammonia production via steam catalytic gasification of microalgae.Results indicated that Ca substitution facilitated the generation of oxygen vacancies.One the one hand,oxygen vacancies could boost the transfer of lattice oxygen,and catalyze water gas shift reaction,thereby promoting the generation of H₂.On the other hand,oxygen vacancies were able to supply sufficient active sites for H-O bond breakage in steam,so that the lattice oxygen could be supplemented by O derived from the H-O bond breaking to maintain the structure of perovskites,and two H atoms combine to generate a significant amount of H₂.An appropriate amount of Ca doping（x=0.1,0.2）enhances the hydrogenation of char-N,the deamination of tar-N,and the conversion of HCN,therefore promoting the NH₃selectivity.However,excessive Ca doping amount（x=0.3）decreases the NH₃ production.La_0.8Ca_0.2Fe O₃ perovskite possesses the highest conversion of nitrogen into NH₃ of 62.2wt.%.Moreover,La_0.8Ca_0.2Fe O₃ perovskite shows superior stability and excellent resistance to coke formation during long-term cycling.